FIG. 9 is a circuit diagram for a conventional LED lighting device. As shown in the figure, the LED lighting device 801 includes an input power supply Vin (DC power supply), an LED drive circuit, and four LED circuits (803a to 803d). The LED drive circuit includes a PWM control IC 804, a switching element 805, a diode 806, a coil 807, resistors Ra to Rd, and a smoothing capacitor 808. The LED circuit 803a is made up of 10 LEDs connected in series between a node a1 and a node a2. Likewise, the LED circuit 803b is made up of 10 LEDs connected in series between a node b1 and a node b2; the LED circuit 803c is made up of 10 LEDs connected in series between a node c1 and a node c2; and the LED circuit 803d is made up of 10 LEDs connected in series between a node d1 and a node d2. The coil 807 has one of its ends connected to the positive terminal of the input power supply Vin. The other end of the coil 807, the anode of the diode 806, and the first conductive terminal (drain) of the switching element 805 are connected to each other. The control terminal (gate) of the switching element 805 is connected to the PWM control IC 804. The second conductive terminal (source) is connected to the negative terminal of the input power supply Vin. The cathode of the diode 806 is connected to the negative terminal of the input power supply Vin via the smoothing capacitor 808. The cathode of the diode 806, a first electrode of the smoothing capacitor 808, and the nodes a1 to d1 are connected to each other. The nodes a2 to d2 are connected to the negative terminal of the input power supply Vin via the respective resistors Ra to Rd. For example, the node a2 is connected to the negative terminal of the input power supply Vin via the resistor Ra. The node b2 is connected to the negative terminal of the input power supply Vin via the resistor Rb. Accordingly, the LED circuits 803a to 803d are connected in parallel between the cathode of the diode 806 and the negative terminal of the input power supply Vin. In addition, the node a2 is coupled also to a current feedback input of the PWM control IC 804.
The drive circuit provides a step-up converter (boost converter). The circuit accumulates energy in the coil 807 (coil) while the switching element 805 is on and applies electromotive voltage derived from that energy along with input voltage across the smoothing capacitor 808 and the LED circuits 803a to 803d while the switching element 805 is off. The current value through the LED circuit 803a is detected using the resistor Ra to make a current feedback to the PWM control IC 804. Based on the feedback, the duty ratio (on/off ratio) of the switching element 805 is controlled. Thus, the current flow through the LED circuit 803a is made constant.
Note that the LED circuits 803a to 803d are in parallel. Therefore, provided that the LED circuits 803a to 803d share the same forward current-forward voltage characteristic (“IF-VF characteristic”), the currents IFa to IFd through the respective LED circuits 803a to 803d are all constant and equal to each other.
FIG. 11 is a circuit diagram for another conventional LED lighting device. As shown in the figure, the LED lighting device 901 includes an input power supply Vin (DC power supply), an LED drive circuit, and LED circuits 903a to 903d. The LED drive circuit includes PWM control ICs 904a to 904d, switching elements 905a to 905d, diodes 906a to 906d, coils 907a to 907d, smoothing capacitors 908a to 908d, and resistors ra to rd. The LED circuit 903a is made up of 10 LEDs connected in series between a node a1 and a node a2. Likewise, the LED circuit 903b is made up of 10 LEDs connected in series between a node b1 and a node b2; the LED circuit 903c is made up of 10 LEDs connected in series between a node c1 and a node c2; and the LED circuit 903d is made up of 10 LEDs connected in series between a node d1 and a node d2.
Separate the FIG. 11 circuitry into four circuits A, B, C, and D. The circuit A consists of components 903a to 908a and ra. The circuit B consists of components 903b to 908b and rb. The circuit C consists of components 903c to 908c and rc. The circuit D consists of components 903d to 908d and rd. Circuits A to D have the same arrangement and operate in the same fashion.
Take the circuit A as an example. The coil 907a has one of its ends connected to the positive terminal of the input power supply Vin. The other end of the coil 907a, the anode of the diode 906a, and the first conductive terminal (drain) of the switching element 905a are connected to each other. The control terminal (gate) of the switching element 905a is connected to the PWM control IC 904a. The second conductive terminal (source) is connected to the negative terminal of the input power supply Vin. The cathode of the diode 906a is connected to the negative terminal of the input power supply Vin via the smoothing capacitor 908a. The cathode of the diode 906a, a first electrode of the smoothing capacitor 908a, and the node a1 are connected to each other. The node a2 is connected to the negative terminal of the input power supply Vin via the resistor ra. The circuitry accumulates energy in the coil 907a (coil) while the switching element 905a is on and applies electromotive voltage derived from that energy along with input voltage across the smoothing capacitor 908a and the LED circuit 903a while the switching element 905a is off. The current value through the LED circuit 903a is detected using the resistor ra to make a current feedback to the PWM control IC 904a. Based on the feedback, the duty ratio (on/off ratio) of the switching element 905a is controlled. Thus, the current flow through the LED circuit 903a is made constant.
The arrangement allows for individual control of the currents IFa to IFd through the circuits A to D. Therefore, even if the LED circuits 903a to 903d do not have exactly the same forward current-forward voltage characteristic (“IF-VF characteristic”), it is still possible to equalize the current flows through the LED circuits 903a to 903d, hence the emitted light intensity (luminance level) from the LED circuits 903a to 903d. 
Related techniques are disclosed in the following publicly known documents: Japanese Unexamined Patent Publication 2004-319583 (Tokukai 2004-319583; published on Nov. 11, 2004), Japanese Unexamined Patent Publication 2002-8409 (Tokukai 2002-8409; published on Jan. 11, 2002), Japanese Unexamined Patent Publication 11-67471/1999 (Tokukaihei 11-67471; published on Mar. 9, 1999), Japanese Unexamined Patent Publication 2004-253364 (Tokukai 2004-253364; published on Sep. 9, 2004), Japanese Unexamined Patent Publication 2004-51014 (Tokukai 2004-51014; published on Feb. 19, 2004), Japanese Unexamined Patent Publication 8-194448/1996 (Tokukaihei 8-194448; published on Jul. 30, 1996), Japanese Unexamined Patent Publication 2000-12907 (Tokukai 2000-12907; published on Jan. 14, 2000), Japanese Unexamined Patent Publication 2004-335443 (Tokukai 2004-335443; published on Nov. 25, 2004), Japanese Unexamined Patent Publication 56-86495/1981 (Tokukaisho 56-86495; published on May 14, 1981), and Japanese Unexamined Patent Publication 59-108297/1984 (Tokukaisho 59-108297; published on Jun. 22, 1984).
Each of the above LED circuits contains 10 series-connected LEDs. These LEDs, although bearing the same product number, may have variations in the forward current-forward voltage characteristic. For example, the VF of LEDs of a specific product number vary from 2.2 volts (minimum) to 2.6 volts (standard) and 3.0 volts (maximum) for the same IF (30 mA) as shown in FIG. 10.
These variations between the LEDs in the forward current-forward voltage characteristic naturally cause variations between the LED circuits containing those LEDs in the forward current-forward voltage characteristic. In other words, in the LED lighting device 801 in FIG. 9, the LED circuits 803a to 803d do not share the same forward current-forward voltage characteristic (“IF-VF characteristic”). In such situations, the LED circuits 803a to 803d pass current of different values, hence emit light of different intensities (luminance levels). These defects can cause problems when the device is applied for illumination purposes and to the area light source in the backlight for large-scale liquid crystal displays. Variations in emitted light intensity can be a cause for uneven luminance. Different current values may lead to differing temperature rises and lifetimes. These problems in turn seriously impact product quality.
In contrast, in the LED lighting device 901 in FIG. 11, the LED circuits 903a to 903d pass current of the same values (IFa to IFd). The device 901 however has disadvantages, such as a high component count, corollary increased chances of malfunction (poor yield), and high manufacturing cost.